<p>Over the past five to ten years, public interest in nuclear energy, decommissioning, and waste management and stewardship has increased, leading to a renewed interest in radioecology (Kuhne 2012), or the study of the relationships between ionizing radiation and the environment (Whicker and Shultz 1982a). Several groups supporting collaborative radioecological research have recently been established, including the European Radioecology ALLIANCE in 2009 (Hinton et al. 2013), the Strategy for Allied Radioecology (STAR) network in 2011 (Kuhne 2012), and the National Center for Radioecology (NCoRE) in the United States in 2011 (Kuhne 2012). The earthquake, tsunami, and subsequent nuclear accident at Fukushima in March of 2011 further emphasized the importance of radioecology in providing timely and technically sound information (such as the transport and fate of radionuclides, potential doses and risks, etc.) for decision making in emergency response as well as in clean up and recovery (Kuhne 2012; Hinton et al. 2013) for both humans and their environment. Although the original and primary aims of the ICRP radiation protection recommendations have been to prevent deterministic effects and minimize stochastic effects to human beings from radiation exposure, the protection framework has recently been extended to include protecting the environment from harmful effects of radiation as well (ICRP 2007, 2008b, 2009).
Radioecology is an interdisciplinary science that encompasses a wide array of topics, including, among others, radiation transport, effects, risk assessment, and remediation (Whicker and Shultz 1982a; Hinton et al. 2013). I consider two topics from different areas of radioecology in this dissertation: radionuclide uptake and dosimetry as well as an assessment of a technique for potential use in remediation.
Part 1 outlines the development of empirical and computational models for prediction of activity concentration and subsequent radiation dose, respectively, in relevant rainbow trout (Oncorhynchus mykiss) organs for selected radionuclides.
Radiation dose rates to biota are typically approximated utilizing dose conversion factors (DCF), which are values for absorbed dose rate per activity concentration in the body or organ (i.e. mGy d-1 per Bq g-1). The current methodology employed by both the International Commission on Radiological Protection (ICRP) and within the Environmental Risks from Ionizing Radiation in the Environment (ERICA) Integrated Approach for calculating dose conversion coefficients is to use Monte Carlo modeling of a homogenously distributed radionuclide within an ellipsoidal phantom chosen to represent a particular organism. However, more accurate estimates can be made based on specific absorbed fractions and activity concentrations.
The first study in Part 1 examines the effects of lake tropic structure on the uptake of iodine-131 (131I) in rainbow trout and considers a simple computational model for the estimation of resulting radiation dose. Iodine-131 is a major component of the atmospheric releases following reactor accidents, and the passage of 131I through food chains from grass to human thyroids has been extensively studied. By comparison, the fate and effects of 131I deposition onto lakes and other aquatic systems has been less studied. In this study we reanalyze 1960s data from experimental releases of 131I into two small lakes and compare the effects of differences in lake trophic structures on 131I accumulation in fish. The largest concentrations in the thyroids of trout (Oncorhynchus mykiss) may occur from 8 to 32 days post initial release. DCFs for trout for whole body as well as thyroid were computed using Monte Carlo modeling with an anatomically-appropriate model of trout thyroid structure. Activity concentration data was used in conjunction with the calculated DCFs to estimate dose rates and ultimately determine cumulative radiation dose (Gy) to the thyroids after 32 days. The estimated cumulative thyroid doses at 32 days post-release ranged from 6 mGy to 18 mGy per 1 Bq mL-1 of initial 131I in the water, depending upon fish size.
The subsequent studies in Part 1 seek to develop and compare different, increasingly detailed anatomical phantoms for O. mykiss for the purpose of estimating organ radiation dose and dose rates from 131I uptake and from molybdenum-99 (99Mo) uptake. Model comparison and refinement is important to the process of determining both dose rates and dose effects, and we develop and compare three models for O. mykiss: a simplistic geometry considering a single organ, a more specific geometry employing anatomically relevant organ size and location, and voxel reconstruction of internal anatomy obtained from CT imaging (referred to as CSUTROUT). Dose Conversion Factors (DCFs) for whole body as well as selected organs of O. mykiss were computed using Monte Carlo modeling, and combined with the empirical models for predicting activity concentration, to estimate dose rates and ultimately determine cumulative radiation dose (?Gy) to selected organs after several half-lives of either 131I or 99Mo. The different computational models provided similar results, especially for organs that were both the source and target of radiation (less than 30% difference between estimated doses). Although CSUTROUT was the most anatomically realistic phantom, it required much more resource dedication to develop than did the stylized phantom for similar results. Additionally, the stylized phantom can be scaled to represent trout sizes whereas CSUTROUT cannot be. There may be instances where a detailed phantom such as CSUTROUT is appropriate, as it will provide the most accurate radiation dose and dose rate information, but generally, the stylized phantom appears to be the best choice for an ideal balance between accuracy and resource requirements.
Part 2 considers the use of reflectance spectroscopy as a remediation tool through its potential to determine plant stress from metal contaminants. Reflectance spectroscopy is a rapid and non-destructive analytical technique that may be used for assessing plant stress and has potential applications for use in remediation. Changes in reflectance such as that due to metal stress may occur before damage is visible, and existing studies have shown that metal stress does cause changes in plant reflectance. The studies in Part 2 further investigate the potential use of reflectance spectroscopy as a method for assessing metal stress in plants.
In the first study, Arabidopsis thaliana plants were treated twice weekly in a laboratory setting with varying levels (0 mM, 0.5 mM, or 5 mM) of cesium chloride (CsCl) solution, and reflectance spectra were collected every week for three weeks using an ASD FieldSpec Pro spectroradiometer with both a contact probe and a field of view probe at 36.8 and 66.7 cm above the plant. As metal stress is known to mimic drought stress, plants were harvested each week after spectra collection for determination of relative water content and chlorophyll content. A visual assessment of the plants was also conducted using point observations on a uniform grid of 81 points. Two-way ANOVAs were performed on selected vegetation indices (VI) to determine the significance of the effects of treatment level and length of treatment. Linear regression was used to relate the most appropriate vegetation indices to the aforementioned endpoints and to compare results provided by the three different spectra collection techniques. One-way ANOVAs were performed on selected VI at each time point to determine which, if any, indices offered a significant prediction of the overall extent of Cs toxicity. Of the 14 vegetation indices considered, the two most significant were the slope at the red edge position (SREP) and the ratio of reflectance at 950 nm to the reflectance at 750 nm (R950/R750). Contact probe readings and field of view readings differed significantly. Field of view measurements were generally consistent at each height.
The second study investigated the potential use of reflectance spectroscopy as a method for assessing metal stress across four different species of plants, namely Arabidopsis thaliana, Helianthus annuus, Brassica napus var. rapa, and Zea mays. The purpose of this study was to determine whether a quantifiable relationship exists between reflectance spectra and lithium (Li) contamination in each species of plant considered, and if such a relationship exists similarly across species. Reflectance spectra were collected every week for three weeks using an ASD FieldSpec Pro Spectroradiometer with a contact probe and a field of view probe for plants treated twice weekly in a laboratory setting with 0 mM or 15 mM of lithium chloride (LiCl) solution. Plants were harvested each week immediately after spectra collection for determination of relative water content and chlorophyll content. Linear regression was used to relate the most appropriate vegetation indices (determined by the Pearson correlation coefficient) to the aforementioned endpoints and to compare results provided by the different spectra collection techniques. Two-way ANOVAs were performed on 12 selected vegetation indices (VI) for each species individually to determine the significance of the effects of treatment level and length of treatment on a species basis. Balanced ANOVAs were conducted across all species to determine significance of treatment, time, and species. LiCl effects and corresponding reflectance shifts were significant for A. thaliana, but Z. mays and H. annuus showed little response to LiCl at the treatment level considered in this study, with no significant differences in relative water content or chlorophyll content by treatment level. B. rapa reflectance spectra responded similarly to Li exposure as Z. mays, but B. rapa did have significant differences in relative water content by treatment level. All species demonstrated a potential stimulatory effect of LiCl, with at least one week of increased reflectance in the near-IR. Different VI proved to be the best predictor of endpoint values for each species, with only SIPI and the ratio of reflectance at 1390 nm to the reflectance at 1454 nm (R1390/R1454) common between species. The most significant VI considering all species together was SIPI, although A. thaliana effects dominate this result. VI determined separately by CP and FOV were occasionally well-related, but this relationship was inconsistent between species, further supporting the conclusion in the previous study that CP and FOV are not interchangeable. These techniques should either be used as compliments or independently, depending on the application.
| Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:3624304 |
| Date | 14 August 2014 |
| Creators | Martinez, Nicole |
| Publisher | Colorado State University |
| Source Sets | ProQuest.com |
| Language | English |
| Detected Language | English |
| Type | thesis |
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